This invention relates to new and useful improvements in the construction of core wraps and shield combinations for telecommunication cables and the like.
Such cables consist of a plurality of pairs of insulated conductors, grouped in a circular formation, and which are wrapped, generally longitudinally, with a core wrap which may be of any suitable plastic or combination of plastic materials which will provide physical, dielectric and thermal protection to the conductors, and/or any combination of the above. The core wrap is lapped over upon itself and is usually secured in place with a helically applied binder.
Over the core wrap, again generally applied longitudinally, is a shield of metallic foil, generally aluminum, which is also lapped over upon itself, and is usually faced on one or both sides with a bonded thermoplastic film which protects the metallic foil from corrosion. This foil laminate provides a protective moisture barrier for the conductors, shields them from external electrical forces, and the thermoplastic film seals to itself at the overlap during the jacket extrusion process, thus improving the moisture barrier.
However, all such cables are susceptible to faults occurring in manufacturing, due to unavoidable weaknesses in the materials and methods used in manufacture, in applying the core wrap, the metallic shield and the outer jacket.
Depending upon the material used, certain plastic films may exhibit excellent dielectric, thermal, and physical strength properties, and others may have excellent bonding and moisture resistant properties or characteristics. Therefore, to produce a cable having all of the above desired operating qualities, it has been found necessary to incorporate multiple laminated core wrap assemblies composed of plastic films having differing physical and electrical properties, to ensure that the core bundle is isolated from the shield. It is further necessary to enclose the aluminum or other metallic foil shield on both sides and edges with a plastic film to prevent corrosion of the metal in the event of moisture ingress through the exterior jacket as a result of mechanical or other damage.
Current cable manufacturing processes generally specify a laminated plastic core wrap, held in place by a helically applied binder, followed by the plastic covered shield, which is formed into a circular or cylindrical shape closely surrounding the core and core wrap, immediately ahead of the jacket extrusion operation. This multiplicity of protective layers creates complications in production, and the presence of binders accentuates the possibility of moisture travel along the cable by capillary action between the core wrap and the shield, which can change the characteristics of the cable and ultimately render it unfit for use.
Furthermore, every different layer of protective material applied to the cable requires additional machinery, and supervision to ensure continual production of quality products, and added maintenance, as the forming portions of the core wrap and shield are critical parts of the cable making process, and must be designed and maintained to very close tolerances.
In addition, present practice provides core wrap and foil shield in the form of spirally wound tapes in the desired width to circumferentially surround the core bundle, and in lengths which can be physically handled without "telescoping". This requires splicing of such materials as a roll of tape or foil comes to an end, and as the manufacture of cable is a continuous process consisting of a simultaneous taping and jacketing which when once started cannot be stopped without ruining the cable, splices must be made to the end of a new roll by the accumulator or "flying splice" method without stopping the cable operation. This can be a critical operation with the possibility of many mishaps occurring within the short time available to make the splice. Hence it is desirable to provide core wrap tapes and foil tapes in as long lengths as possible, or in other words, as great a diameter roll as can be handled without damage.
Still further, if the shield is protected with a layer of thermoplastic material, for corrosion resistance, such thermoplastic layer may bond to the jacket of the cable during the jacket extrusion process, causing difficulties in some cases in stripping the jacket. This may be ameliorated by providing a protective layer on the foil shield which does not bond to the jacket material, but such a construction fails to provide a sealing of the shield protective material to itself, which may contribute to cable failure.